1972 Puerto Rico DC-7 crash
A DC-7CF similar to the accident aircraft
|Date||31 December 1972|
|Summary||Single engine failure on take-off|
|Site||Pinones, near Isla Verde |
|Aircraft type||Douglas DC-7CF|
|Operator||American Express Leasing|
|Registration||N500AE (NTSB listed as previous N23N)|
|Flight origin||San Juan-Isla Verde International Airport (SJU/TJSJ), Puerto Rico|
|Destination||Managua Airport (MGA/MNMG), Nicaragua|
The 1972 Puerto Rico DC-7 Crash was an aviation accident that occurred on December 31, 1972, in Carolina, Puerto Rico. It is most notable for causing the death of Pittsburgh Pirates star Roberto Clemente. Due to inadequate maintenance, engine no. 2 failed after takeoff. After initiating a turn to return to the airport, the aircraft then eventually descended into or attempted to ditch into the ocean a mile offshore. All two passengers and three crew were killed.
Roberto Clemente was a baseball star for the Pittsburgh Pirates, winning two World Series championships with them, and becoming only the 11th player in Major League Baseball history to collect 3,000 hits on September 30, 1972, in his final at-bat. In October 1972, he went to Managua, Nicaragua to coach the Puerto Rico national baseball team during that year's Baseball World Cup.
Many countries sent help and this made Clemente desire to send help, paid for with his own money, and to personally supervise the delivery of goods. Clemente had been convinced to involve himself in the relief efforts by local television show host, reporter and celebrity Luis Vigoreaux. Clemente had already sent three cargo planes and a ship to help in relief efforts.
The previous planes reported that the military had taken the relief cargos. Clemente suspected profiteering by the military and chartered a fourth plane to go to personally stand up to the military dictator for the people because he thought as a celebrity he couldn't be touched.
The accident caused the deaths of all five people on board, including Clemente. The airplane crashed immediately after take-off from Isla Verde International Airport, flying into the ocean at the adjacent area known as Piñones.
The cargo carrier
Clemente and a relief committee had leased the aircraft for $4,000 ($22,800 US2020) from a local airline, American Air Express Leasing Company, owned by a 27 year old Puerto Rican named Arthur J. Rivera.
Unknown to Clemente or the pilot, the aircraft, a four engined Douglas DC-7 (DC-7C), had suffered a non-fatal taxiway accident just 29 days before the fateful flight took place. This accident damaged the No. 2 and 3 propeller blades and the No. 3 engine cooler scoop. Rivera was advised to replace one of the engines—he pressed his mechanics to do what they could to inspect the engine and keep it in service. The mechanics inspected the engines and could not find a reason to justify replacing one. Protocol on a piston engine after a sudden stop is to disassemble the engine to maganaflux the parts for cracks; that was not done. An FAA maintenance inspector inspected (later the report says witnessed the test) the propeller shaft limits after the sudden stoppage repairs and found them within tolerances.
In the 1970s the post-war era of cargo carriers operating with surplus piston driven prop planes was at its end as high costs of maintenance could not keep up with newer aircraft technology. Rivera had just regained from the FAA his ability to operate a cargo plane by claiming it was his only livelihood. He was cutting corners as he struggled to keep American Air Express Leasing going against a tide of change in the airline industry.
On December 31, bad weather hit the area near Isla Verde International Airport. Despite this and warnings by his father, his wife Vera and son Roberto, Jr. that they had bad feelings and premonitions about the flight, Clemente insisted on going on with the relief mission and taking off on that day. Clemente himself is reported to have had a dream about overlooking his own funeral days before.
After volunteers spent most of the afternoon loading the aircraft, pilot Captain Jerry Hill boarded the plane as the sole member of the flight crew. Owner Rivera sat in the co-pilot's seat though he was only certified to fly the twin-engine Douglas DC-3 which had a single row radial engine). Mechanic Francisco Matias sat in the flight engineer's seat because a flight engineer could not be found.
Mr. Matias was simply a mechanic employed by another airline that was moonlighting with several other mechanics on cargo carriers at the same airport; he had taken the place of another mechanic that could not fly that day. Several attempts had been made by Mr. Rivera and Captain Hill to secure a flight engineer.
Captain Jerry Hill, a well-qualified, seasoned pilot, was in command. The pilot was found by chance several days earlier while watching the plane being loaded. After another pilot had failed to show from a waitlist of itinerant pilots, Captain Hill flew back from Miami on short notice. He sat in the plane for the first time the previous morning of the flight—sleeping all day in a crew bunk to rest for the flight.
This was the maiden flight for this aircraft since Rivera had purchased it several months previously and it was the first time the pilot had flown with either Rivera or Francisco. Clemente boarded with associate Angel Lozano around the same time as the aircraft's crew.
Aircraft loaded weight
At takeoff, National Transportation Safety Board (NTSB) investigators later estimated the Aircraft gross weight at 148,943 pounds with crew, fuel, and freight based on a fuel receipt and customs declaration. The plane had been loaded by a ground crew led by a qualified loadmaster.
- Plane 72,763 pounds
- Fuel 32,830 pounds (round-trip)
- Cargo 38,288 pounds (from flight plan)
- Crew (5) 1,000 pounds (200 pounds per person)
- Fuel reserve (1-hr) 4,063 pounds
At the estimated 148,943 pounds, the aircraft was 3 percent (4,193 pounds) over the 144,750-pound maximum takeoff weight for a DC-7C. The crew submitted a flight plan with a cargo weight of 38,288 pounds (including crew and reserve fuel); without fuel, that is a takeoff weight of 116,110 pounds—under the 144,750-pound limit. At a minimum, the 1,420-mile one-way flight would need 14,240 pounds of fuel—that would be a takeoff weight of 130,350 pounds and again under the limit. With a one-way fuel load, they would have to divert to someplace like San José, Costa Rica or Panama City to refuel for the return leg. Possibly the fuel for the flight was being donated and there would be a challenge finding fuel in a now destroyed Nicaragua capital city. This left the investigators to guess how much fuel was actually on board; secondary documents indicated that enough fuel for a round-trip was purchased and presumably loaded onto the plane, "The actual weight and balance computation made by the crew was not found."
Grayzone of weight
The commercial airline industry of the 1970s used general factors in calculating takeoff weight and maximum takeoff weight. Using the current understanding moves the plane weight within a gray zone of maximum takeoff weight.
How much fuel was loaded onto the aircraft was a focus of conjecture. The investigation found evidence the aircraft was fueled for a round trip. The investigators calculated the weight of that fuel and pushed the aircraft weight to 148,943 pounds. The 1,420-mile flight to Nicaragua was less than half the 3,605-mile range of a fully loaded DC-7C which holds 7,825 gallons of fuel. At takeoff, the plane was filled to 60 percent of fuel capacity. However, the 1972 investigation was limited, in 1972 the effect of temperature on fuel density and hence fuel weight was not well understood by the airline industry. Depending on temperature, gasoline ranges from under 6 pounds per gallon to 6.75 pounds per gallon at 60 °F. The fuel needed for the four-hour flight to Nicaragua and four-hour return to Puerto Rico was somewhere between 28,480 and 32,400 pounds, a 3,900-pound difference. Puerto Rico has a hot climate, in December the temperature is above 80 deg F; the fuel would have actually weighed on the low side. The estimated 4,193 pounds the flight was overweight is equivalent to 707 gallons of fuel which is 9 percent of a DC-7s fuel capacity; sufficient fuel for an hour of flight.
Another concept introduced after the 1970s is a Zero-fuel weight; the weight of fuel in the wings has less of a structural effect than weight in the fuselage—modern planes have a zero-fuel weight that allows for increasing the maximum takeoff weight when that weight is in fuel.
Air density affects the maximum takeoff weight. Colder air provides more buoyancy, more engine performance, and a wider safe engine operating band. The later takeoff time placed the aircraft in the 10 deg F cooler 76 deg F evening air which being denser provided both better buoyancy and increased engine performance which being at sea level provides a considerable increase to the aircraft's weight capacity (a DC-7C sized aircraft could see a 7,000 pound change with a 10 deg drop in air temperature).
The flight might not have been overloaded after all.
Regardless, the pilot, Captain Hill, would have known neither the fuel density concept nor the zero-fuel weight concept nor air buoyancy adjustment though he should have become familiar with the engine performance change just through flight time. The NTSB investigators found that while weight was a factor in the accident it was not the cause of the accident.
A more interesting calculation is that the plane could not have landed with both a full cargo load and enough fuel for a return trip; if at takeoff the aircraft was 148,943 pounds, then in Nicaragua it would have landed after burning 14,240 pounds of fuel and would weigh 134,700 pounds which is 25,700 pounds over the landing weight limit. To land at a specified weight and have return fuel would limit the cargo to 12,600 pounds.
However, the overweight landing is also a grayzone. The takeoff and landing limits are commercial peacetime limits, there are 'war emergency' load limits that are 20% higher—the war emergency takeoff limit would be 178,000 pounds and the landing limit would be 160,000; the DC-7C was within those limits which are set for new military aircraft receiving military maintenance. Captain Hill would have been aware of these emergency cargo limits from his time as a U.S. Air Force Major flying the Douglas C-124 Globemaster II on a trans-Pacific route (possibly Douglas C-74 Globemaster). If the Nicaraguan Earthquake relief was an event allowing for emergency cargo limits is an open question.
Pragmatically, Clemente's wife provides a first-hand account, "Mrs. Clemente said she was concerned that the plane seemed old and overloaded."
The weight calculation by the investigation left open questions.
Oversight of safety
After the weight discrepancy, a question that became the topic of a lawsuit was who was responsible to stop this plane from taking off. There is the air traffic control and there is the Federal Aviation Administration (FAA).
The air traffic control has a responsibility to direct traffic where and when to fly which means they do not determine if a flight should fly. That leaves the FAA.
In the lawsuit, the decision was that the FAA is not liable for allowing flight N500AE to takeoff. In those arguments, it appears the FAA was motivated to present an overloaded aircraft condition over a mechanically unsound aircraft condition. The question of an incomplete aircrew was somewhere in-between. The court stated it is ultimately the responsibility of the pilot, however, the court did not find the pilot at fault.
With respect to the pilot while they are responsible by tradition, pragmatically it would be difficult to hold pilots strictly responsible under inadequate preflight preparation and/or planning. For example, there are latent defects in the engine and in adherence to an engine maintenance schedule. These latent issues cannot reasonably be discoverable by a pilot, particularly when pilots are often itinerant to a specific plane and even carrier. The pilot had, in this case, aborted a takeoff and returned the plane for three hours of spark plug reconditioning on discovering a latent defect in the maintenance and tuneup of the right-side engines (engine No. 3 was later found to have been unable to produce full power even with clean sparkplugs).
The court's decision was focused on the question before them which was if the FAA was responsible to inform the public with knowledge they held—the court decided that because the FAA had not inspected planes at that airport previously, then despite their knowledge of the condition of the aircraft and failure to act, they did not have that responsibility.
Regardless of the actual weight and who is responsible, well after a 5:59 P.M. New Year's Eve sunset and on a moonless night (twilight ended at 7:20 P.M.), at 9:11 P.M. local standard time—after the previously aborted takeoff and additional mechanical work—the plane taxied around the airport's runway 7. By then the weather had cleared and visibility was at 10 miles and only a few clouds were visible.
After engine run-up by the crew, the flight was cleared for takeoff at 9:20:30 P.M. for the four-hour flight to Nicaragua. On takeoff, the aircraft took an exceptionally long takeoff roll and gained very little altitude. A left turn was commenced towards the North, after which, at 9:23:15 P.M., the San Juan Tower received the following transmission: "N500AE coming back around." The pilot would have had to begin a fuel dump to reduce the aircraft weight to allow landing. The reduced fuel load would bring the plane within the allowed weight; a typical dump rate is one to two tons of fuel per minute. To land, they needed to first dump 32,000 pounds of fuel which would take from sixteen to thirty-two minutes.
Either at the last radio transmission or soon thereafter the plane experienced a catastrophic failure of engine number 2.
It is possible that engine number 3 was also lost. Nos. 2 and 3 are closest to the fuselage. The No. 2 and 3 engines contain the hydraulic pumps. If both were lost that would render the pilot reliant on a controls reversion system. With reduced control and possibly loss of electrical, the pilot was then faced with the challenge of ditching the aircraft into the sea while maintaining a relation to the horizon over water on a moonless night. In this scenario, the aircraft was essentially unflyable.
With an engine lost, the airplane slowly descended and about ten to thirty seconds later crashed into the Atlantic Ocean at a point approximately 1.5 miles offshore, and 2.5 miles on the 040 degree radial from the western end of Runway 25. In that time, 500 to 2,000 pounds of fuel would have dumped. In the last seconds of flight, Ground effect (aerodynamics) would have kept the plane aloft skimming the wave tops.
The mechanic that was unable to fly witnessed the takeoff from the ground at the airport, Mr. Delgado Cintrón testified that on takeoff the engines sounded pretty (even and normal). However, the plane was too low at 25 feet off the ground. Other witnesses estimated the plane gained altitude to 100 feet. After the aircraft was out of sight behind trees the engines sounded good and then a few seconds later he heard three backfires. He heard a large explosion which he thought was the impact with the ocean followed by silence.
Issues with engine design
That a DC-7 had lost an engine on takeoff was not unexpected. During World War II, twice as many aircraft were lost to the engine that powered the DC-7 and wartime aircraft like the Boeing B-29 Superfortress as were lost to enemy action.
The Wright R-3350 that powered these planes had started as a problem multi-row radial design that was rushed to wartime production. In post-war civilian use, those problems continued and given the option of jet engines, aircraft with this engine were less favored for commercial flights and often converted to cargo planes.
Clemente would have no reason to have known the history of the R-3350 but Captain Hill would have had a good understanding from over 12,000 flight hours piloting multi-row piston-powered radial engined aircraft over his nearly thirty-year career including the DC-4, DC-6, DC-7, C-46, and the USAF Globemaster.
A basic surface area issue of cooling additional rows of radial cylinders in multi-row radial engines was understood and well known. A key concern was a lean air/fuel condition causing detonation due to the high supercharger boost on these engines which by itself and with cooling issues was dangerous. Problems with cooling had plagued this engine since its use in the hot Pacific tropics during the War—resulting in adding an air scoop to the top of the engine cowling that directed air to cool the back cylinders. Those problems only added to the engine's sensitivity to detonation, particularly on takeoff.
Recovery efforts started almost immediately after the aircraft went down. By 11 PM, radio and television stations across Puerto Rico were informing the public about the accident. A crowd formed around Pinones beach, many of them trying to help search efforts. Of the five people on board, only Hill's body was recovered.
The wreckage site was not discovered until January 4, 1973, due to extremely rough surface conditions and poor underwater visibility.
On or after January 7, divers from a United States Navy ship reported that the aircraft wreckage was scattered throughout the bottom of the ocean at a depth of 100 to 130 feet, in an area of approximately 4 acres. The aircraft was broken into several sections, most of them badly crushed or demolished. Both wings were separated from the fuselage. The cockpit area forward of the main junction box was destroyed and the instrument panel and mechanical controls missing. The nose gear assembly was retracted. All four engines were accounted for, but none of them were found attached to the wing structure. Two of the engines were together at a distance of approximately 200 feet from the right wing, which itself was upside down on the left side of a fuselage section.
Three engines were recovered from the ocean floor on January 11, 1973; including Nos. 2 and 3. A review of the engine log books showed they had received 100 hour inspections four and five months earlier and previously to being purchased by Rivera; all spark plugs on engines Nos 3 and 4 and a few cylinders on engine Nos 2, 3, and 4 were replaced. On the previous flight in September the No. 3 engine was shut down and feathered due to sparkplug fouling
- The rings were found intact in the cylinders of No. 3 engine, however, repeatedly sparkplugs had fouled in the previous months and likely would have fouled before completion of the planned flight.
- The 32 sparkplugs were undamaged with normal gaps.
- The 16 cylinders contained no damage.
- The crankshaft was broken and deeply wrinkled and slightly twisted; it was not determined if this was on or prior to impact (under full power on entrance to water and water resistance on prop caused the crankshaft to twist until breaking)
- The sumps contained a thick black sludge (including pieces from previous engine repairs).
- The propeller of the No. 2 engine was feathered, indicating there was full engine failure at some point before the crash and the pilot had been able to respond to this. No. 2 engine was internally destroyed.
- The number 16 cylinder was destroyed. The two sparkpugs were bent and coated in oil.
- All 16 connecting rods were broken (connects the pistons to the crankshaft).
- All the cylinder skirts were bent (indicates crankshaft continuing to turn).
- The sumps contained a thick black sludge.
- The No. 1 engine showed no damage.
- The sparkplugs had no fouling and the electrode gaps were normal.
- The valves and pistons were undamaged.
- The sumps contained a thick black sludge.
Part of the fuselage and the tail of the airplane were also found.
In the simplest explanation, the FAA concluded that after a failure of one engine the plane had inadequate power to maintain altitude during a turn (insinuating they could not dump fuel fast enough to achieve a weight to power ratio that allowed level flight). After a few miles, eventually the plane flew into the ocean on a moonless night. The lack of a horizon possibly prevented the pilot from realizing the altitude loss from only 100 feet over the ocean which would have indicated a need to prepare for a water landing with reduced engine power on splashdown (engines appear to have been at full power and fuselage damage indicates an aircraft at higher speed).
An unexplored scenario is that following an engine loss the pilot began fuel dumping to lighten the aircraft. The prop wash dispersed the fuel and given the low elevation formed a thermobaric weapon which was ignited by the burning engine. The resulting airburst destroyed the aircraft. The DC-7 contained fuel dumping facilities. A hazard of fuel dumping is ignition of the fuel and precautions are taken to eliminate all sources of ignition, and also to prevent turbulence that mixes the fuel with air. Aircraft typically don't explode on impact and in this case the plane would have been more or less in level controlled flight into the water. An airburst explains the explosion heard after an engine fire was seen and the heavy destruction of the fuselage.
A scenario that seemed to be considered by the FAA Investigation but later dismissed was a load shift on turning which careened the plane into a wing strike with water which cartwheeled the plane along the surface. Two engines were found several hundred feet directly ahead of the wing, indicating a level entry into the water.
The cause of the crash could not be determined precisely due to difficulties recovering the wreckage. Probable causes were attributed to lean detonation, poor maintenance, excessive wear in engine components, engine damage from a previous taxiing accident that was not repaired, uncertified co-pilot, uncertified flight engineer, the aircraft possibly being 4,000 pounds overloaded with fuel, and inadequate crew preparation in correcting the above.
National Transportation Board findings:
Complete power loss—complete engine failure/flameout - 1 engine (No. 2)
Partial power loss—partial loss of power - 1 engine (No. 3) (presumably this is inferred from there should be adequate power on three engines for flight which it was not)
- powerplant (failed due to engine damage from sudden stoppage during a taxiway accident 12/2/72)
- miscellaneous acts, conditions - No. 3 excessive-wear/play (unable to develop full power)
- personnel - maintenance, servicing, inspection: inadequate maintenance and inspection
- pilot in command - inadequate preflight preparation and/or planning
- personnel - operational supervisory personnel: deficiency, company maintained equipment, services, regulation
- miscellaneous acts
Remarks: Flight engineer unqualified (later court proceedings clarified there was no evidence that Matias was sitting in as a flight engineer which would mean there was no flight engineer).
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NTSB Report: F.A.A. accident report dated February 22, 1973, Identification NYC73AN094 File 3-3757 (the full narrative is not available, as of 2019, all 1973 accident dockets have been destroyed per the National Transportation Safety Board Records Management Division 490 L'Enfant Plaza East, SW Washington, DC 20594 )